How the Decking Calculator Works

Enter your deck length and width, choose your board and pick timber or composite. The calculator works out board runs from laid width of each board including its gap, joist quantities from correct centres for your material, then adds fixings and wastage.

The laid width is key concept. A standard 145mm deck board with a 5mm expansion gap covers 150mm per run, which works out at roughly 6.7 linear metres of board per square metre of deck. A 4m x 3m deck therefore needs around 80 linear metres of board before wastage or about 88 metres with 10 percent added.

The Subframe Carries Everything

Boards get attention but subframe decides how deck feels and how long it lasts. The standard build uses 47 x 100mm C16 or C24 treated joists, stepping up to 47 x 150mm for longer spans or raised decks, supported on posts set in postcrete or on adjustable pedestals over hard surfaces.

Joist spacing depends on what goes on top:

• Timber boards: 400mm centres
• Composite boards: 300mm centres, because composite flexes more between supports
• Diagonal board patterns: close the centres by around 100mm from the figures above

Skipping this adjustment is classic composite mistake, producing a deck that feels spongy underfoot within a season. Add doubled joists under board joints, noggins between joists at 1.2m intervals for rigidity and a slight fall away from the house for drainage.

Boards, Gaps and Fixings

Expansion gaps are not optional. Timber and composite both move with moisture and temperature, so leave 5 to 6mm between board edges and follow manufacturer’s end gap spec for composite, which expands along its length far more than timber. Fixings run at 2 screws per board per joist crossing, so a 4m board crossing 11 joists takes 22 screws and a modest deck swallows screws by the thousand. Use coated decking screws for treated timber and hidden clip systems composite ranges are designed around.

On durability, check treatment class when buying timber, boards should be treated to Use Class 3 for outdoor use above ground, while any subframe timber near or touching ground should be Use Class 4. The subframe is the part you cannot replace without lifting the deck, so this is wrong place to save money.

Worked Example

A 4m x 3m deck in 145mm timber boards on a 400mm centre subframe:

• Board runs across the 3m width: 3,000 ÷ 150 = 20 runs of 4m
• Board metres: 80 linear metres, plus 10 percent = 88m, so twenty two 4m boards
• Joists across the 4m length at 400mm centres: 11 joists at 3m
• Posts: 9 on a 3 x 3 grid for a low level deck, each with postcrete
• Screws: 20 boards x 11 joists x 2 = 440, so a 500 box minimum
• Add perimeter joists, noggins and a few spare boards for future repairs

For diagonal laying, take wastage to 15 or 20 percent because every board ends in an angled cut.

Planning Rules for UK Decking

Decking is permitted development in most gardens provided the platform sits no more than 30cm above ground level, combined with other outbuildings and extensions, covers no more than half garden. Rise above 30cm, perhaps on a slope and planning permission enters the picture along with balustrades for safety. Leave a 150mm gap below the damp proof course where a deck meets the house and ventilate beneath boards so the subframe can dry, since a sealed void is what rots decks from underneath.